As an ion sensor measuring ion concentration in a solution, an ion selective electrode is given, and is important and known widely in the field of electrochemistry. From the beginning of the 20th century, a pH electrode in which glass is employed as a pH sensitive film has been known, and is used widely presently. A liquid film type ion selective electrode in which an ion sensitive matter is dispersed with a polymer such as PVC (polyvinyl chloride) and a plasticizer has been used from the 1960s. The ion selective electrode is used as a sensor measuring ion concentration of K+, Na+ and the like in blood in a an analytical instrument for clinical examinations. The ion sensor has been reported by many literatures (see Non-patent Literature 1).
As shown in FIG. 10, an ion selective electrode 50 comes into contact with a test solution 51 to be examined through an ion-sensitive film 52, and an internal solution 53 as a standard and an electrode 54 are provided in the ion selective electrode 50. As an internal electrode, an Ag/AgCl electrode is employed.
A coated wire was reported as an ion selective electrode in which an ion-sensitive film is directly formed on an Ag/AgCl electrode without any internal solution (see Non-patent Literature 2).
There is dry chemistry in which all internal solution is solidified and formed sheet-like. For example, Fuji DRI-CHEM of Fujifilm Corporation (DRI-CHEM is a trademark) was reported (see Patent Literature 1).
There have been many trials for miniaturizing an ion sensor, and an ISFET ion-sensitive field effect transistor) was reported in which an ion-sensitive film is formed on a gate of a FET (field-effect transistor). Accordingly, a multi sensor in which various kinds of ion-sensitive films are integrated to one sensor substrate so as to measure various ions simultaneously has been reported (see Non-patent Literature 3).
Especially in recent, for easing instability of a complete individual ion sensor such as the coated wire, various ion sensors in which conductive polymer films are compounded with ion-sensitive films have been reported (Non-patent Literature 4). The ion sensor exchanges ions for electrons between an electrode material and the ion-sensitive film and is reported to be effective for stabilizing the potential in the long run. As the conductive polymer film, polypyrrole, polythiophene, polyaniline, poly(3,4-ethylenedioxythiophene): PEDOT, poly(3-octylthiophene):POT and the like are known, and the conductive polymer film stabilizes the contact potential with the electrode material such as silver, gold, platinum and glassy carbon. In this case, as a method for using the conductive polymer film, the method that the conductive polymer film is formed between the electrode material and the ion-sensitive film and the method that the conductive polymer film is mixed with the ion-sensitive film are given.
Such an ion sensor measures the potential of the ion-sensitive film which is changed corresponding to the concentration of ion to be measured in the solution. The standard potential must be required for measuring the potential, and a reference electrode 56 is employed which is connected through a potentiometer 55 as shown in FIG. 10. Therefore, for miniaturizing the ion sensor, there is a problem of miniaturization of the reference electrode, and several arts for miniaturizing the reference electrode have been reported (see Non-patent Literature 5).